The Three-dimensional Velocity Structure Study Before And After The Impoundment Of The Xiaowan Reservoir

Xiaowan reservoir is located in the Yunnan area where more earthquake happens,with high magnitude,high altitude and high intensity. The dam is height of 292 meters, the normal storage water level is 1240 meters and the total capacity is about 15 billion cubic meters. After the impoundment on16 December 2008,earthquakes occurred along the Lancang river and Heihui river,and these increasing earthquakes mainly concentrated in two clusters, indicating obvious reservoir-induced seismicity(RIS) phenomena,and so the RIS problem in high intensity area is worth to study. Previous researches of the Xiaowan reservoir have more focused on the geological structure, focal mechanism,seismicity and seismic sequence, and now it is still lack of the research results of the crustal velocity structure based on the seismological method. As the Xiaowan reservoir earthquake monitoring network had been finished since 21 May 2005, before and after the impoundmentr, it recorded rich and continuous seismic data and provided us a good opportunity to research the upper crust velocity structure in different water filling periods of the Xiaowan reservoir. The crust velocity structure study is the foundation to explore the RIS genetic mechanism, which has great significance for the reservoir prediction.The paper used the selected 780 earthquakes to make the inversion of ‘minimum 1-D velocity’by the Kissling method firstly. And then we use the ‘minimum 1-D velocity’ as the initial velocity model, to research the crust medium in different water filling periods of the Xiaowan reservoir area with the method of simultaneous inversion of earthquake hypocenters and velocity structure, so we can get the difference distributions map of earthquake relocation,3-D P wave,S wave velocity structure and Vp/Vs value. Finally the detailed analysis and discussed on the basis of the research joint with the geological structure,water level and the lithology information of Xiaowan reservoir area, and here are some conclusions:1、The Xiaowan reservoir has experienced five stages, and the seismic activity space distribution image changed significantly before and after the impoundment of the Xiaowan reservoir. Before the impoundment, most earthquakes were located near the Xucun hydropower station region where is the origin of the Heihui river and the southern of the Ruili-Longling fault. Only a few earthquakesoccured along the Lancang river and the Heihui river. But after impoundment, a large of earthquakes occurred at the Dasi area(the first cluster) on the Lancang river after the second filling and the Niujie area(the second cluster) on the Heihui river after the third filling when the elevated water flowded respectively, and those earthquakes concentrated in two clusters.2、After the impoundment, the seismicity of the Xiaowan reservoir area show strong correlation with the water level. As the water level rise, the frequency increased; on the other hand, the frequency declined. So the characteristic of reservoir induced earthquake is obvious. The two cluster earthquakes occurred fastly when the backwater arrived, so the induced seismicity of the Xiaowan reservoir belongs to ‘quick response type’.3、The ‘minimum 1-D velocity’ in the Xiaowan reservoir area is derived by the Kissling method.After the thirteen interactions,and the RMS residual of the minimum 1-D velocity decrease from 0.81 s to 0.12 s, drop rate of 85.19%,and the data variances decrease from 1.64s2 to 0.04s2, drop rate of97.56%. By the comparision the earthquakes relocation RMS residual of the initial velocity model and the minimum 1-D velocity model, we found out that the ‘minimum 1-D velocity’ can significantly improve the quality of earthquake location.4、Before the impoundment, the intersecting distribution of high or low value for P wave velocity and Vp/Vs value shows the complexity and the lateral heterogeneities of the structure within several kilometers near the Xiaowan hydropower station. Before and after impounding, the P wave velocity of the Xiaowan reservoir area has good consistency, while the S wave velocity has obviously low speed abnormality. These abnormalities manly concentrated within 10 km of the Lancang river and the Heihui river in the horizontal space,and appeared has different performance in the depth.The low Vs and high Vp/Vs value anomaly in the first cluster can only extend down to 5km, it may be correlated well with the granite developed relatively complete below the Lancang river reservoir, and so water is not easy to infiltrate. The low Vs and high Vp/Vs value anomaly in the second cluster area can extensively extend down to 5km,and continue to reach 14 km individual place, it is likely to be the depth which can penetrate in the area. With the increase of storage time, the form of the high Vp/Vs value has changed.5、Most earthquakes in the Xiaowan reservoir area distribute along the fault on either side, and appear to be a belt in the depth direction.Most tectonic earthquakes before impoundment were located between 8km and 14 km. No earthquakes bigger than ML3.0 occurred in the focus area. After the impoundment, the RIS in the first cluster on the Lancang river is distributed below the high Vp/Vs region between 0km and 5km.We speculate that it is due to the strong pressure caused by the reservoir water load have change the physical characteristicsof the media below 5km, which led to a lot of earthquakes in the area. Most of the RIS in the second cluster on the Heihui river is distributed inside or at the edge of the area with high Vp/Vs value between 0km and 10 km, a few RIS can extend down to 15 km, deeper than the 10 km of general RIS in the lowintensity area. It should be due to the Weixi-Qiaohou fault located in the high mountains area can cut into the crust deeperly.